Intermolecular electron-transfer reactions involving hydrazines

Article abstract of DOI:10.1021/jo9510090

The self-exchange electron-transfer (ET) rate constant k₂₂ for 1,2,3,4,5-pentamethylferrocene was determined by NMR line broadening to be 8.5(±0.8) × 10⁶ M^-1 s^-1 (25 °C, CD₃CN/0.09 M Et₄NBF

Full text of DOI:10.1021/jo9510090

J . Org. Chem. 1996, 61, 1405-1412
1405
In ter m olecu la r Electr on -Tr a n sfer Rea ction s In volvin g Hyd r a zin es
Stephen F. Nelsen,*,† Ling-J en Chen,† Michael T. Ramm,† Gilbert T. Voy,† Douglas R. Powell,†
Molly A. Accola,‡ Troy R. Seehafer,‡ J obiah J . Sabelko,‡ and J ack R. Pladziewicz*,‡
S. M. McElvain Laboratories of Organic Chemistry, Department of Chemistry, University of Wisconsin,
Madison, Wisconsin 53706-1396, and the Department of Chemistry, University of Wisconsin,
Eau Claire, Wisconsin 54702
Received J une 2, 1995X
The self-exchange electron-transfer (ET) rate constant k22 for 1,2,3,4,5-pentamethylferrocene was
determined by NMR line broadening to be 8.5((0.8) × 106 M-1 s-1 (25 °C, CD3CN/0.09 M Et4NBF4)
and k11 for the bis-N,N-bicyclic hydrazine, 9,9′-bi-9-azabicyclo[3.3.1]nonane, to be 8.7(0.5) × 103
M-1 s-1 (25 °C, CH2Cl2). These self-exchange rate constants are used to analyze cross reactions of
these molecules with hydrazines, ferrocenes, and tetramethyl-p-phenylenediamine (TMP D) using
Marcus theory. Cross-reaction rate constants for eight reactions are reported. Included are six
cross-reactions between methylferrocenes and four cyclic hydrazines, one hydrazine, hydrazine-
reaction, and the reduction of TMP D+ by a cyclic hydrazine. These are the first cross-reaction
rate constants reported for hydrazine-hydrazine and hydrazine-TMP D+ ET reactions. The cross-
reaction rate constants are used to test the application of Marcus theory to hydrazine ET reactions
and to extract estimates of hydrazine self-exchange ET rate constants in systems for which direct
measurement is presently impossible.
In tr od u ction
bridgehead carbons on each side of the dinitrogen bridge
common to both rings of the hydrazines, with u designat-
ing an unsaturated bridge and Cp *, Cp ′, and Cp penta-
methyl, monomethyl, and unsubstituted cyclopentadienyl
ligands, so that the compound designations convey easily
The Marcus cross-relation1 estimates rate constants for
electron transfer (ET) between different species 1 and 2
by using the equilibrium constant K12 of (1) and the
K12
[1]° + [2]•+ y z [1]•+ + [2]°
(1)
average of the intrinsic barriers (2), which, when used
with the rate expression (3) for the effect of ∆G° for (1)
λ12 ) (λ11 + λ22)/2
(2)
(3)
k ) Z12 exp(-[∆G° + λ12]2/4λ12RT)
remembered structural information. The self-ET rate
constants, k11, for the hydrazines were measured by slow
exchange region NMR,3 and k22 for ferrocene derivative
self-exchange have been studied by fast exchange region
NMR. We used data from Weaver and co-workers for
Cp *2F e and Cp 2F e4 and interpolated k22 for the other
two ferrocenes. k12 values were measured by stopped-
flow spectrophotometry, monitoring the absorption of the
ferrocenium species. The hydrazine k11 and ferrocene k22
values are very different, k22(Cp *2F e)/k11(21/u 22) ) 1.3
× 104, probably largely as a result of their very different
Marcus inner sphere vertical ET barriers,2 λin, which are
argued to be approximately 2 kcal/mol for ferrocenes4 and
measured at 37.6 kcal/mol for an N,N′-bis-bicyclic hy-
drazine using a dimeric compound (see below). For the
hydrazine-ferrocene reactions reported,3 k12(obs) is within
a factor of 4 of that calculated, k12(calc), using (4) with
the traditional Z12 ) 1 × 1011 M-1 s-1 and self-exchange
rate constants from NMR measurements. This agree-
ment is comparable to the best-behaved inorganic sys-
tems.2
on the effective barrier, produces (4) as the expression
for the cross rate constant where k12(calc). These expres-
k12(calc) ) (k11k22K12f12)1/2
(4)
(5)
2
ln f12 ) (ln K12)2/4 ln(k11k22/Z12
)
sions clearly work rather well for many inorganic reac-
tions.2 We recently examined these relationships for ET
between bis-N,N′-bicyclic (sesquibicyclic) tetraalkyl-
hydrazines 22/u 22 and 21/u 22 and substituted ferrocenes
Cp *2F e, Cp *Cp F e, and Cp ′2F e.3 In abbreviating these
In the present work we determined k22 for Cp *Cp F e0/+
,
structures we use the number of carbons connecting the
finding it to be significantly lower than the extrapolated
value previously employed. This is important because
this ferrocene has a reduction potential ideally suited to
reactions with many organic molecules of interest, and
† UWsMadison.
‡ UWsEau Claire.
X Abstract published in Advance ACS Abstracts, February 1, 1996.
(1) Marcus, R. A.; Sutin, N. Inorg. Chem. 1975, 14, 213.
(2) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811,
265. (b) Sutin, N. Prog. Inorg. Chem. 1983, 30, 441.
(3) Nelsen, S. F.; Wang, Y.; Ramm, M. T.; Accola, M. A.; Pladziewicz,
J . R. J . Phys. Chem. 1992, 96, 10654.
(4) (a) McManis, G. E.; Nielson, R. M.; Gochev, A.; Weaver, M. J . J .
Am. Chem. Soc. 1989, 111, 5533. (b) Nielson, R. M.; McManis, G. E.;
Safford, L. K.; Weaver, M. J . J . Phys. Chem. 1989, 93, 2152.
0022-3263/96/1961-1405$12.00/0 © 1996 American Chemical Society

1406 J . Org. Chem., Vol. 61, No. 4, 1996
Ta ble 1. Ra te Con sta n t Da ta for Cp *Cp F e0/+ Mixtu r es in Aceton itr ile Con ta in in g 90 m M Et4NBF 4 a t 25 °C
ring hydrogen signala methyl signalb
Nelsen et al.
øP, 10-3
CP, mM
WDP, Hz
kex × 10-6
vDP, Hz
øP, 10-3
CP, mM
WDP, Hz
kex × 10-6
c
c
no.
Ctot, mM
ν
DP, Hz
1
2
3
4
5
77.1
73.3
69.9
66.8
63.9
930.4
940.5
951.0
960.2
3.09
5.19
7.36
9.27
11.57
0.24
0.38
0.51
0.62
0.74
4.10
6.54
8.91
11.51
13.77
6.34
7.05
7.86
7.65
8.68
440.8
418.9
397.6
376.0
355.3
2.95
5.06
7.10
9.17
11.15
0.23
0.37
0.50
0.61
0.71
8.21
13.61
19.40
26.22
30.99
7.72
8.50
8.62
8.46
9.11
1115
a
b
WD ) 0.26, WP ) 644 ( 40 Hz. νD ) 915.4, νP ) 5 749 ( 10, ∆ν ) 4834 ( 10 Hz. WD ) 0.47, WP ) 413 ( 20 Hz. νD ) 471.6, νP ) -9
962 ( 10, ∆ν ) 10 440 ( 10 Hz. c M-1 s-1
.
knowing k22 allows greater confidence in analysis of
reactions that utilize it or its radical cation. We have
also considerably widened the range of structural varia-
tion of the compounds used, including hydrazines which
have lone pair-lone pair dihedral angle θ values of 180°
and near 120°, as well as the radical cation of the
aromatic diamine tetramethyl-p-phenylenediamine,
TMP D, which significantly extends the k22 range for the
oxidants used. This is of significance because it allows
a more rigorous test of (2)-(5).
Resu lts
Self-Exch a n ge Ra te Con sta n ts. We determined k22
for Cp *Cp F e0/+ using fast exchange region NMR. The
rate constant for fast exchange region NMR line broad-
ening kinetics is given by (6),4b,5 where øD and øP are the
F igu r e 1. Plot of kex vs NMR linewidth for mixtures of
Cp *Cp F e0 and Cp *Cp F e+. The circles are for ring and the
squares for methyl hydrogen signals.
kex ) 4πøDøP(∆ν)2/[(WDP - øDWD - øPWP)Ctot
]
(6)
that exchange between the radical and diamagnetic forms
is rapid compared to the difference in the NMR frequency
and also that electron spin relaxation is faster than the
hyperfine frequency in the radical, so that the spin
exchange is a two-site problem.5 Heisenberg spin ex-
change between the radicals can make electron spin
relaxation fast enough so that (6) holds, its rate depends
upon radical concentration,6 and the hyperfine frequen-
cies are different for the methyl and ring hydrogen
signals. The methyl and ring hydrogen signals give
different kex values at low concentration and hence low
WP, indicating that spin exchange between the radicals
may not be rapid enough for (6) to hold. We suggest that
the higher, limiting kex values measured at higher WDP
are likely to be correct and evaluate kex(CD3CN, 25 °C)
for Cp *Cp F e0/+ as 8.5((0.8) × 106 M-1 s-1 from these
data.
mole fractions of the diamagnetic and paramagnetic
species D and P respectively, ∆ν ) |νD - νP| (hertz) is
the chemical shift difference between the NMR signals
of D and P, WD and WP are the NMR line widths at half-
height in hertz for these species, WDP is the linewidth
for the D,P mixture, and Ctot is the sum of the concentra-
tions of D and P. Only small amounts of the paramag-
netic species can be added, and øP was determined using
(7).4b Table 1 summarizes our results for Cp *Cp F e0/+
.
øP ) (νDP - νD)/∆ν (7)
Four determinations of the chemical shifts and line
widths for Cp *Cp F e+BF 4 between 0.03 and 0.12 M in
-
acetonitrile gave the values and ranges for WP and νP
quoted at the bottom of Table 1, without a clear trend
with concentration (signal-to-noise is a problem for small
concentrations of Cp*CpFe+, more serious for the broader
and less intense ring hydrogen signal). Cp *Cp F e0/+
mixtures have two DP signals (ring and methyl hydro-
gens) which should produce the same øP and kex values.
Slightly different values are produced using (6) and (7),
presumably reflecting experimental errors. The kex
values are quite insensitive (((0.01 × 106) M-1 s-1) to
the observed uncertainty in νP and vary by e((0.08 ×
106) M-1 s-1 (e(1%) and by ((0.5-0.7) (e(8%) for those
in WP(Me) and WP(ring), respectively. The ring hydrogen
signal is seen from Table 1 to produce slightly smaller
Hydrazines have far smaller self-exchange rate con-
stants than ferrocenes, but some may be studied in the
slow exchange NMR region. Measurement of k11 for
0/+
33N)2 proved difficult for solubility reasons. Neutral
33N)2 was too insoluble in acetonitrile, dimethylform-
amide, or nitromethane to allow observation of slow
exchange region NMR kinetics, which requires a large
excess of neutral over radical cation. Its solubility is
higher in methylene chloride, and k11(25 °C, CD2Cl2) was
determined to be 8.7(0.5) × 103 M-1 s-1. Solvent effect
studies on k11 have been carried out for 22/u 220/+ and
k
ex values than the methyl hydrogen signal, and there is
drift to higher kex values at higher CP (and hence WDP
)
in both data sets, but less for the methyl signal. The
plot of kex vs WDP shown in Figure 1 suggests that the
assumptions involved in deriving (6) are not fulfilled at
the lower concentrations studied. Equation 6 assumes
(5) Chen, M.-S.; De Roos, J . B.; Wahl, A. C. J . Phys. Chem. 1973,
77, 2163.
(6) Hausser, K. H.; Bunner, H.; J oachims, J . C. Mol. Phys. 1966,
10, 233.

Intermolecular Electron-Transfer Reactions Involving Hydrazines
J . Org. Chem., Vol. 61, No. 4, 1996 1407
∆Gq11(calc),c
Ta ble 2. Cr oss Ra te Stu d ies of Hyd r a zin e ET
b
10-5k12
,
k11(calc),c
reaction
∆G° a
M-1 s-1
M-1 s-1
kcal/mol
Z12, effectd
1. Cp ′2F e+,22/tBu Me0
2. Cp *Cp F e+,22/tBu Me0
3. TMP D+,22/tBu Me0
4. Cp *Cp F e+,22/tBu iP r 0
5. Cp *2F e+,22/tBu iP r 0
-3.9
-0.3
-0.2
-5.2
+0.2
-1.0e
-5.4e
-3.1
-0.5
3.2((0.3)
0.23((0.02)
1.1((0.1)
8.1((0.8)
0.14((0.03)
0.59((0.06)
0.59((0.06)
11((1)
22
36
5.6
23
9.6
15.6
15.3
16.4
15.6
16.1
14.9e
18.9e 2
13.4
14.7
+0.15,-0.08
<0.01
<0.01
+0.26, -0.12
<0.01
<0.07
6. Cp *Cp F e+,22/iP r 2
75e
0.08e
990
109
0
6a. Cp *Cp F e+,22/iP r 2
0
7. Cp *Cp F e+,33N)2
+0.14, -0.06
<0.02
0
8. 21/21+,33N)2
0.0209f
0
a
b
Unit, kcal/mol, using data from Table 3. At 25 °C in acetonitrile, µ ) 0.1 M. c For the neutral in each reactant pair, calculated from
k12 and (4), using data from Table 3 and Z12 ) 1 × 1011 M-1 s-1
.
Fractional change in k11(calc) using Z12 ) 1 × 109 M-1 s-1 (positive
d
number) and Z12 ) 1 × 1013 M-1 s-1 (negative number). e For E°(22/iP r 2) ) +0.08 V. The numbers in italics (6a) use E°′ of -0.11 (see
text for why this was done). f ((0.0005).
21/u 220/+,7 giving k11(CD3CN)/k11(CD2Cl2, 25 °C) ratios
Ta ble 3. F or m a l P oten tia ls a n d Self-ET Ra te Con sta n ts
Eo,a
V vs SCE
k11(NMR),b
∆Gq11(NMR),
of 0.205 and 0.199. Our best estimate for k11(CD3CN, 25
°C) for 33N)20/+ is therefore 20% that in CD2Cl2, or 2.2 ×
103.
Stop p ed -F low k12 Mea su r em en ts. The one-electron
oxidations of three 2,3-diazabicyclo[2.2.2]octane deriva-
tives with unlinked 2,3-dialkyl substituents, abbreviated
22/RR′, have been examined by reacting them with three
couple
M-1 s-1
kcal/mol
Hydrazines
22/u 220/+
-0.241
+0.058
+0.11
-0.10
-0.01
+0.01
12.1((1) × 103
2.29((.10) × 103
(too slow)
11.89 ( 0.05
12.87 ( 0.02
21/u 220/+
22/tBu Me0/+
22/tBu iP r 0/+
(too slow)
0/+
33N)2
2.17 × 103 c
12.9
11.63 ( 0.02
21/210/+
18.5((0.5) × 103 d
Ferrocene Derivatives
-0.109
+0.124
+0.281
+0.395
Cp *2F e0/+
Cp *Cp F e0/+
Cp ′2F e0/+
Cp 2F e0/+
2.9 × 107
8.5 × 106
8.3 × 106
8.1 × 106
7.27
8.00
8.01
8.02
Aromatic Amine
TMP D0/+
+0.12
1.46 × 109 d
4.95
ferroceniums and with TMP D+ (Table 2, Reactions 1-6).
a
b
0
In acetonitrile containing 0.1 M nBu4NClO4 at 25 °C. In
acetonitrile at 25 °C, without supporting electrolyte for the
hydrazines and with 0.09 M Et4NBF4 for Cp *Cp F e. c From ref
10. c Estimated from the rate constant in CD2Cl2 (see text).
The one-electron oxidation of 33N)2 has been studied
using Cp *Cp F e+ and 21/21+ as oxidants, monitoring
d
Extrapolated to 25 °C from the activation parameters reported
in ref 19.
decreases in k22 with Cp 2F e+ concentration increases in
the absence of added electrolyte.8 The measured value
for k22(Cp *Cp F e0/+) is 57% of that used previously,3
which was interpolated from the values measured by
disappearance of Cp *Cp F e+ (reaction 7) and formation
of 33N)2 (reaction 8). All eight reactions are observed
Weaver and co-workers for Cp *2F e0/+ and Cp 2F e0/+
.
+
Weaver and co-workers pointed out that k22 values for
metallocenes do not correlate with E°′ for these com-
pounds,4 so perhaps we should not be surprised that our
interpolation between the k22 values for ferrocene and
decamethylferrocene was not very successful for
Cp *Cp F e. They attribute the substantial decrease in
k22 between cobaltacenes and ferrocenes to smaller
electronic matrix coupling elements V (which is equal to
half the energy separation between the ground state and
excited state surfaces at the electron transfer transition
state, and is called H12 by Weaver and co-workers and J
or Hab by others2) in the iron systems, caused by greater
localization of the odd electron at the metal.4a They also
evaluate “closest approach” V° values of 0.2 and 0.1 kcal/
mol for Cp *2F e0/+ and Cp 2F e0/+, respectively, using a
solvent friction-electron tunneling model. Because the
steric effect of methylation would only seem capable of
lowering π,π overlap at the transition state, methyl
substitution presumably would have to be increasing odd
electron density in methylated (Cp *) rings relative to the
unmethylated (Cp ) rings, to account for the increase in
k22 for Cp *2F e and larger V value. The NMR spectra of
to obey second-order rate laws at 25 °C, µ ) 0.10 in
acetonitrile, and the observed second-order rate con-
stants, k12, are listed in Table 2, and for convenience, the
formal potentials and self-ET rate constants necessary
for analysis are collected in Table 3.
Discu ssion
k
22 for Cp *Cp F e0/+. Because of its convenient formal
potential, E°′, Cp *Cp F e0/+ was employed as an oxidant
for many of the compounds studied, making experimental
determination of its k22 value especially important for
interpretation of the stopped-flow studies. Weaver and
co-workers found a significant effect of the presence of
added electrolyte for Cp 2F e0/+ but little dependence of
-
k22 upon ionic strength in the region of 0.1 M total BF4
concentration.4b Hunt and co-workers report larger
(7) (a) Nelsen, S. F.; Wang, Y. J . Org. Chem. 1994, 59, 1655. (b)
22/u 230/+ was also studied,5a giving k11(CD3CN)/k11(CD2Cl2, 25 °C) )
0.284. Analysis of the rate constants for the 22/u 230/+ system was only
consistent with
a
∼30% smaller ∆G*out than for the other two
compounds, which is not consistent with the small changes in molec-
ular size. We concluded that this system has a problem, most likely
cation stability, because 22/u 23+ is noticably less stable in the presence
of the neutral hydrazine than are the other two, and that the solvent
rate ratio is likely to be not as reliable.
(8) Kirchner, K.; Dang, S.-Q.; Stebler, M.; Dodgen, H. W.; Wherland,
S.; Hunt, J . P. Inorg. Chem. 1989, 28, 3604.

1408 J . Org. Chem., Vol. 61, No. 4, 1996
Nelsen et al.
Ta ble 4. Com p a r ison of Rin g Ca r bon π Sp in Den sities
depend upon hνin, so different Zii values might be
expected to occur for different compound types. It is
therefore important to consider what Zii values might be
expected for self-ET reactions of the compound types
studied here, as well as how large the effects of different
for Meth yla ted a n d Un m eth yla ted F er r ocen iu m Rin gsa
π
compound
δP(CH) δP(CH3) FCπ(CH) FCπ(CMe)
∑FC
Cp 2F e+
+28.67
-0.017
-0.011
-0.17
-0.16
-0.19
Cp *Cp F e+ +19.36
-41.73
-38.44
-0.021
-0.019
Cp *2F e+
Z
12 values for cross-reactions between different compound
types might be on the k11 values calculated using (4) with
a
From NMR data in CD3CN at 25 °C, using a(H)/δP ) 73.27
the traditional Z12 value of 1 × 1011 M-1 s-1
.
G/ppm10a and the simple McConnell relationships.10b
Zii estimates for self-ET reactions can be obtained using
Hush theory,12 from the transition energies Eop for
dimeric radical cations 1-4. Marcus and Sutin define
the ferrocene cations bear directly upon this question,
because the π spin density at the ring carbons, FCπ, can
be calculated from the ESR splitting constants a(H),
which are available from the paramagnetic shifts.9 FC
π
values using chemical shifts from Weaver and coworkers4a
for Cp *2F e+ and Cp 2F e+ 10 and data from Table 1 for
Cp *Cp F e+ are compared in Table 4. The total π spin
density at the ring carbons is similar for all three
compounds, and although the Cp * ring has almost twice
the spin density as the Cp ring for Cp *Cp F e+, FCπ is only
7% higher for the Cp * ring of Cp *2F e+ than for the Cp
ring of Cp 2F e+. We conclude that there is not a correla-
tion between k22 and FCπ for these ferrocenes. We do not
know why k22 for Cp *Cp F e0/+ is closer to that of Cp 2F e0/+
than of Cp *2F e0/+, but the differences are rather small,
and more than one factor might contribute.
Cr oss Ra te Stu d ies. The cross relations (2)-(5)
traditionally used for analysis of ET reactions between
two different species assume that the rate constants will
be controlled by the ET barrier (λ12 + ∆G°), that λ12 will
be given by (2), and that the same preexponential factor
(Z12 in (3)) may be used for all cases. More modern ET
theory2 has emphasized that ET reactions will have
different preexponential factors depending upon whether
they are adiabatic or diabatic (nonadiabatic). The pre-
exponential factor for adiabatic reactions is approxi-
mately νn ) chνin[λin/(λout + λin)]1/2 in a semiclassical model
where single solvent and inner sphere barrier crossing
frequencies are assumed,2 where hνin is the frequency (in
cm-1) of the nuclear vibration which leads to crossing the
activated complex configuration. The preexponential
factor for diabatic reactions is controlled by the electronic
coupling frequency νel, which is approximately given by
(1.5 × 1014)[V2/λ1/2] at 25 °C.2 Intermolecular self-ET has
been argued to be diabatic because of small V values for
all three compound types studied here, hydrazines,7
ferrocenes,4 and TMP D.11 With the large range in
structure for the compounds employed, substantial varia-
tions in hνin certainly occur. Ferrocenes have effective
hνin estimated at under 400 cm-1, hydrazines doubtless
have several effective hνin values but are believed to have
intermediate frequencies active for ET, and TMP D has
larger, aromatic stretching frequencies involved (Grampp
and J aenicke estimated hνin at 1668 cm-1).11 Tunneling
effects also appear in the preexponential factor and
E
op as λ for such symmetrical “intervalence” compounds,2a
although the solvent reorganization contribution to λ, λout
,
can obviously be different for the self-ET transition state
and the dimeric species. Table 5 summarizes Zii value
estimations from the Eop data. The solvent dependence
of Eop for 1 gives a 9.0 kcal/mol λout(CH3CN),13 which is
close to the 10 kcal/mol estimated from solvent depend-
ence of k11 for self-ET of sesquibicyclic hydrazines5a and
we employ 46-49 kcal/mol as a reasonable range for λ
for 22/220/+ (the bicyclic rings of neutral 22/22 may be
slightly more twisted than those of 1, which may raise λ
somewhat). 214 is included despite the fact that k11 for
22/tBu Me has not been experimentally determined.
Using a k11 value range derived from the cross rate
studies in this work, we determine that a Z11 value
similar to those for the sesquibicyclic hydrazines apply
to 22/tBu Me as well. Determination of Zii for 3 uses the
λout estimate from the solvent dependence of Eop.15 We
agree with Weaver and co-workers that λout for 3 should
be reasonably close to that for Cp 2F e0/+, but not on the
partitioning of λout and λin for these systems.15b 4 is not
as good a model for TMP D as the other dimeric cases
(9) (a) For a recent paper on obtaining a(H) from NMR data, see:
Petillo, P. A.; De Felippis, J .; Nelsen, S. F. J . Org. Chem. 1991, 56,
6496. (b) a(HR) = QHFCπ, a(Hâ) = QMe cos2 θ FCπ, where cos2 θ ) 0.5
for a methyl group. We employed QH ) -23 and QMe ) +54.8c (c)
Fischer, H. Free Radicals; Kochi, J . K., Ed.; Wiley: New York, 1973;
Vol. 2, Chapter 19, p 435.
(10) (a) Fritz, H. P.; Keller, H. T.; Schwarzhaus, K. E. J . Organomet.
Chem. 1967, 7, 105. (b) Rettig, M. F.; Drago, R. S. J . Am. Chem. Soc.
1969, 91, 1361, report a(H) ) +0.183 for 6+ using the above data,8a
which we do not understand because both the paramagnetic shift and
the FCπ Fritz and co-workers report correspond to a(H) ) +0.338 G.
(11) (a) Grampp, G.; J aenicke, W. Ber. Bunsenges. Phys. Chem. 1991,
95, 904. (b) Grampp and J aenicke9a suggest V to be as small as 0.1-
0.02 kcal/mol for TMP D, but we have argued on the basis of optical
studies of the 4 radical cation that their V was significantly under-
estimated because of underestimation of λin.14
(12) For a review, see: Hush, N. S. Coord. Chem. Rev. 1985, 64,
135.
(13) Nelsen, S. F.; Adamus, J .; Wolff, J . J . J . Am. Chem. Soc. 1994,
116, 1589.
(14) Nelsen, S. F.; Chang, H.; Wolff, J . J .; Adamus, J . J . Am. Chem.
Soc. 1993, 115, 12276.

Intermolecular Electron-Transfer Reactions Involving Hydrazines
J . Org. Chem., Vol. 61, No. 4, 1996 1409
Ta ble 5. Zii Va lu es for Self-ET (25 °C, CH3CN) Estim a ted
fr om Eop Va lu es of Dim er ic Com p ou n d s a n d kii for
Self-ET
tively, but that the reactions involving Cp *2F e (entries
1 and 4) give substantially lower k11(calc), ratios 9 and
16, respectively. We do not know why the use of Cp *2F e
gives smaller k11(calc) values than the other ferrocenes,
but note that smaller k12 when Cp *2F e is employed than
with Cp *Cp F e is also observed for the other reaction
studied, Table 2, reaction 5. It seems possible that the
smaller k12 than expected might result from a steric
effect: methyls block approach to the ferrocene iron on
both sides of Cp *2F e. However, if the methyl groups
present steric hindrance to electron transfer, it is not
apparent in the self-exchange rate constants because
Cp *2F e has the largest k22 of the ferrocenes used.4
Relatively few hydrazines have isolable radical cations,
making preparation of solutions with accurately known
radical cation concentrations difficult, and because most
hydrazine radical cations decompose in the presence of
excess neutral compound, accurate measurement of k11
by dynamic NMR broadening is often impossible. Sig-
nificant line broadening of the neutral species NMR by
the radical cation is not observed when k11 is below about
Eop
,
λ
out(est),
kii,
λ(est), Zii(est)
dimer‚+ kcal/mol kcal/mol monomer M-1 s-1 kcal/mol × 10-11
1a
2b
3c
4d
46.6
52.2
21.4
29.7
9
9
11
22/22
7 × 102
46-49 2-7
52-54 0.7-3
b
22/tBu Me 20-36
Cp 2F e
8.1 × 106 20-22 0.4-0.9
unknown TMP D
1.5 × 109 20-25 70-600
a
b
Reference 13. Reference 14. The k11 range uses k11(calc)
values from Table II. c Reference 15. Reference 16.
are for their monomers, both because the charge-bearing
units are less similar (AM1 calculations suggest that λin
might be 2.4 kcal/mol less for TMP D itself than for the
six-membered-ring-containing 4)16 and because the λout
values are expected to be greater for the extended
geometry of 4 than for the π-overlapping geometry argued
for the TMP D0/+ transition state;11 we include a wider
range for the λ22 estimate for TMP D than for the other
compounds for these reasons. It should be noted that
our estimate (based both on the Eop for 4 and more recent
theoretical estimates of λin for TMP D) is larger than that
employed by Grampp and J aenicke.11 We conclude from
the information in Table 5 that 1011 M-1 s-1 is a
reasonable estimate of the effective Zii for both hydra-
zines and ferrocenes, but that it is larger for TMP D.
We note that a k11(calc) value obtained from an
observed k12 and an experimental k22 value using (2)-
(5) is quite insensitive to the Z12 value employed unless
K12 is large. The effect of using Z12 of 109-1013 M-1 s-1
on k11(calc) for our data is shown in Table 2, last column
(and Table 6, see below), where it can be seen that it only
becomes significant when ∆G° becomes more negative
than about -2 kcal/mol and never exceeds a 50% change
in k11(calc) for any of our data. We conclude that k12
values are not expected to be influenced significantly
compared to experimental error by rather wide variations
in the effective preexponential factor for the cross ET
reaction and that the use of (2)-(5) for estimation of k11
is reasonable, even for compounds of very different
structure.
The lower experimental k22 value for Cp *Cp F e0/+ has
caused us to revise our estimate of k22 for Cp ′2F e0/+ to
between those of Cp *Cp F e0/+ and Cp 2F e0/+. Our new
estimate for Cp ′2F e0/+ is the same as the experimental
value reported by Wahl and co-workers (although their
number was measured at about 3 mM cation concentra-
tion in the absence of added electrolyte).17 Consequently,
we have recalculated the previously reported3 self-
exchange rate constants k11(calc), see Table 6. It may
be noted that k11(NMR) is now rather close to k11(calc)
for the reactions of Cp ′2F e+ and Cp *Cp F e+ with 21/
u 220, k11(NMR)/k11(calc) ratios of 1.4 and 2.0, respec-
300 M-1 s-1
. We hoped to use cross rate constant
measurements to estimate k11 for 22/RR′ hydrazines in
this work. These compounds have θ near 120°, and as a
result, k11 values which are too small to measure by NMR
line broadening. The 22/tBu Me and 22/tBu iP r k11(calc)
values obtained using ferrocenes (Table 2, entries 1, 2,
and 4, 5, respectively) show good internal agreement,
especially when the smaller values produced using
Cp *2F e, noted above, are considered. The k11(calc) values
are also consistent with our inability to observe NMR line
broadening for 22/tBu iP r 0/+ and 22/P r 20/+ mixtures,18 an
increase expected in λin for these θ ∼120° compounds
compared to θ ∼0 hydrazines, and the increase in Eop for
2 compared to 1 (∆Gq11(calc) for 22/tBu Me0/+ is 2.0 and
1.7 kcal/mol higher than ∆Gq11(NMR) for 22/220/+ using
the two ferrocenes, while ∆Eop/4 for 2 and 1 is 1.4 kcal/
mol). Only a slightly smaller k11(calc) value is obtained
from the Cp *Cp F e+,22/tBu iP r 0 reaction than from the
Cp *Cp F e+,22/tBu Me0 reaction, indicating that the effect
of additional steric hindrance at the nitrogens when the
bulkier iPr group is present is small.
The Cp *Cp F e+,22/iP r 2 reaction (entry 6) is of par-
0
ticular interest because of the anomalous CV at a
platinum electrode for 22/iP r 20/+, where the oxidation and
reduction peaks have a large separation.18 We argued
that this behavior is caused by isopropyl group gearing
effects which produce the radical cation in an unstable
isopropyl group rotamer, which rapidly isomerizes to the
more stable form on the CV time scale, so that the
conformations of the neutral form which is oxidized and
the radical cation which is reduced are different.
A
significant part of our argument was that the heteroge-
neous rate constant for electron exchange, ks, should be
similar for compounds as similar as 22/RR′. Independent
measurement of solution ET kinetics involving these
compounds is important because a direct relationship is
(15) (a) McMannis, G. E.; Gochev, A.; Nielson, R. M.; Weaver, M. J .
J . Phys. Chem. 1989, 93, 7733. (b) Although the Eop vs solvent polarity
parameter γ plots for the radical cations of Cp 2F e values linked by
zero, one, and two acetylenes (3 is linked by one acetylene) all
extrapolate to similar values at γ ) 0 (11.4, 10.1, and 10.2 kcal/mol
respectively), Weaver and co-workers specifically reject assuming this
is λin for Cp 2F e0/+, because they believe this number to be <2.0 kcal/
mol on theoretical grounds. Although we agree that the enthalpy
contribution to λin should be small, it is not as obvious that λin is that
small. Use of Weaver and co-worker’s interpretation requires a λout of
about 20 kcal/mol for Cp 2F e0/+, which which we believe is too large,
considering the similar size of Cp 2F e, sesquibicyclic hydrazines, and
TMP D.
predicted between homogeneous and heterogeneous ET
barriers.2 The E°′ value for 22/iP r 2
is in question
0/+
because of the large oxidation and reduction potential
peak separation observed at a Pt electrode. The gearing
hypothesis would make the midpoint of the oxidation and
reduction CV curves different from E°′, which is a relative
measure of the free energy difference between relaxed
(16) Nelsen, S. F.; Yunta, M. J . S. J . Phys. Org. Chem. 1994, 7, 55.
(17) Yang, E. S.; Chan, M.-S.; Wahl, A. C. J . Phys. Chem. 1980, 84,
3094.
(18) Nelsen, S. F.; Chen, L.-J .; Petillo, P. A.; Evans, D. H.; Neuge-
bauer, F. A. J . Am. Chem. Soc. 1993, 115, 10611.

1410 J . Org. Chem., Vol. 61, No. 4, 1996
Ta ble 6. Bis-N,N′-bicyclic Hyd r a zin e, Su bstitu ted F er r ocen e Cr oss Ra te Stu d iesa
Nelsen et al.
c
10-5k12
,
k11(calc)d
k11(NMR)/
11(calc)f
e
reaction
∆G° b
M-1 s-1
(∆Gq
)
k
Z12 effectg
11
1. Cp *2F e+,22/u 220
2. Cp ′2F e+,21/u 220
3. Cp *Cp F e+,21/u 220
4. 21/u 22+,Cp *2F e0
-3.0
-5.1
-1.5
-3.9
23((2)
1410 (13.2)
1630 (13.1)
1170 (13.3)
143 (14.5)h
8.6 (1.3)
1.2 (0.2)
2.0 (0.4)
16.0 (1.6)h
+0.19, -0.07
+0.44, -0.16
+0.03, -0.02
+0.26, -0.10
63((1.3)
3.5((0.5)
13.5((2.3)
a
b
d
Data of ref 3 reanalyzed using the data of Table 3. kcal/mol. c At 25 °C in acetonitrile, µ ) 0.1 M. In M-1 s-1 for the hydrazine,
calculated from k12 and (4), using the data of Table 3 and Z12 ) 1 × 1011 M-1 s-1
.
e In kcal/mol. f The number in parentheses is ∆Gq11(calc)
- ∆Gq11(NMR), kcal/mol. Fractional change in k11(calc) using Z12 ) 1 × 109 M-1 s-1 (positive number) and Z12 ) 1 × 1013 M-1 s-1
g
h
(negative number). Calculated for the hydrazine.
cation and neutral. As shown in Table 2, if the true E°′
were the +0.08 V value which fit the time dependence of
the CV experiments assuming gearing, k11(calc) is similar
for the three Cp *Cp F e+,22/RR′0 reactions studied (Table
2, reactions 2, 4, 6: k11(calc) for 22/iP r 2 is 2.1 times that
of 22/tBu Me and 3.3 times that of 22/tBu iP r ), while if
of Cunkle.20 The syn and anti NN bond rotational
E°′ were the -0.11 value which also fit the CV experi-
isomers of 32N)2+ have different oxidation potentials and
ments assuming “direct” oxidation with no gearing but
a small ks value for 22/iP r 20/+, k11(calc) for 22iP r 2 would
be 1/450th that of 22/tBu Me (Table 2, entry 6a). The
syno + anti+ yk2z syn+ + antio
(8)
k-2
latter interpretation appears unreasonable given the
small k11(calc) change between 22/t Bu Me and 22/
t Bu iP r ; k11 for 22/R R′0/+ is rather clearly not as sen-
sitive to steric effects as the “direct oxidation” hypothesis
would require. The difference between the pathways is
clearer in the cross rate study than in the CV study
because heterogeneous ET halves the barrier com-
pared to homogeneous ET, making the difference between
the two mechanistic interpretations of the CV data
smaller.
UV spectra, so their mixtures can be analyzed by both
CV wave size and UV spectrophotometry. The second-
order rate constants for the exchange reaction (8), called
k2 and k-2 previously,20 were found to affect both the CV
curve for its oxidation (because they allow conversion of
syn+ to the more stable anti+ by (8) so that the relative
size of the oxidation waves of syn+ and anti+ depends
upon scan rate) and the rate of isomerization of photo-
chemically enriched syn+ mixtures to anti+ by addition
of neutral compound. Best fit to the CV curves as a
function of scan rate at 22 °C gave k2 ) 260 M-1 s-1, k-2
) 4500 M-1 s-1, while that to the UV spectrophotometry
isomerization data at 11 °C (where thermal conversion
of syn+ to anti+ is negligible) gave k2 ) 105 M-1 s-1, k-2
) 2000 M-1 s-1. Assuming that self-ET for syno/+ and
antio/+ have the same rate constant (which seems plau-
sible to us), cross rate theory gives k11(calc) for 32N)2 )
We used the aromatic amine radical cation TMP D+
as the oxidant for 22/tBu Me0 in Table 2, reaction 3. This
considerably extends both the range of structure of the
oxidant and the range of k22 values examined. Grampp
and J aenicke determined k22(25 °C, acetonitrile) for
TMP D0/+ to be 1.46 × 109 M-1 s-1 by fast exchange region
ESR studies,19 which is 172 times faster than that for
Cp *Cp F e0/+, and as noted above, Z22 appears to be
significantly larger than for hydrazines and ferrocenes.
The k11(calc) ratio for Cp *Cp F e+ vs TMP D+ oxidation
of 22/tBu Me0 (Table 2, reactions 2 and 3) is 6.4, which
is significantly poorer agreement than is seen using
Cp ′2F e and Cp *Cp F e. We believe that seeing as small
a deviation as observed from the behavior predicted by
(4) for compounds with structures and electron transfer
properties as different as TMP D and 22/tBu Me (k11 ratio
4 × 107) is notable confirmation of its utility.
1110 M-1 s-1 at 22 °C (∆Gq ) 13.16 kcal/mol) from the
11
CV experiment, while it is 470 M-1 s-1 at 11 °C (∆Gq
)
11
13.13 kcal/mol) from the spectrophotometric experiment.
Although measured by very different types of experi-
ments, these k11 values are in rather good agreement both
with each other and with the k11 value estimated by NMR
line broadening and ferrocenium oxidation for 33N)2.
Whether similar k11 values are expected for 32N)2 and
33N)2, however, requires consideration of whether the
internal geometry reorganization values (λin) are similar
for these two compounds, especially since changing
bicyclic ring size by one carbon leads to significant rate
differences for sesquibicyclic compounds. Semiempirical
calculations do not do a good enough job on these
compounds to make consideration of calculated values
The k11(calc) value from the 33N)2 ,Cp *Cp F e+ reaction
0
(990 M-1 s-1, Table 2, reaction 7) and the value of 2170
M-1 s-1 estimated from NMR line broadening experi-
ments in CH2Cl2 give a k11(NMR)/k11(calc) ratio of 2.2,
corresponding to ∆∆Gq11(calc) of 0.5 kcal/mol. This is not
much higher than the ∆∆Gq11(calc) values of 0.2 and 0.4
kcal/mol obtained for 21/u 22 using Cp ′2F e+ and
Cp Cp *F e+ (Table 6) and demonstrates that the special
conformational restrictions of bis(N,N′-bicyclic) structures
are not necessary for simple cross rate theory to provide
useful estimates of k11 for hydrazines. Information on
ET rate constants for the structurally similar but con-
formationally more complex bis-N,N-bicyclic θ ) 180°
hydrazine 32N)2 which are independent of both the NMR
and stopped-flow methods are available from the studies
+
of λin fruitful. Although 33N)2 was the first hydrazine
radical cation isolated,21a the crystal structure originally
obtained of its PF6- salt21b had undetected disorder which
made the nitrogens come out planar and significantly
underestimated the NN bond length. This result unfor-
tunately clouded our understanding of the geometry
change upon electron loss from hydrazines until a non-
(20) Nelsen, S. F.; Cunkle, G. T.; Evans, D. H.; Haller, K. J .; Kaftory,
M.; Kirste, B.; Clark, T. J . Am. Chem. Soc. 1985, 107, 3829.
(21) (a) Nelsen, S. F.; Kessel, C. R. J . Am. Chem. Soc. 1977, 99, 2392.
(b) Nelsen, S. F.; Hollinsed, W. C.; Kessel, C. R.; Calabrese, J . C. J .
Am. Chem. Soc. 1978, 100, 7876.
(19) (a) Grampp, G.; J aenicke, W. Ber. Bunsenges. Phys. Chem. 1984,
88, 325, 335.

Intermolecular Electron-Transfer Reactions Involving Hydrazines
J . Org. Chem., Vol. 61, No. 4, 1996 1411
cations,23 which allowed measurement k12 for its oxida-
tion by 21/21+ by following the increase in absorption at
340 nm (Table 2, reaction 8). The k11(calc) value for
33N)2 obtained from the hydrazine-hydrazine reaction
is a factor of 9.1 smaller than that from the hydrazine-
ferrocene reaction and 20-fold smaller than k11(NMR).
Ta ble 7. Com p a r ison of Neu tr a l a n d Ra d ica l Ca tion
X-r a y Geom etr ies a t Nitr ogen
33N)2
neutrala
rad.cat.b
difference
d(NN), Å
1.505(3)
109.2, 108.1
106.7(1)
108.0
1.357(4)
118.7, 118.9
111.4(2)
116.3(3)
180
-0.148 (9.8%)
R(CNN), deg
R(CNC), deg
R
av,c deg
+8
θ,d deg
180
Con clu sion s
32N)2
e
e, f
d(NN), Å
R(CNN), deg
1.469(2)
111.1(1)
111.2(1)
101.3(1)
107.9(1)
180
1.323(4)
120.7(1)
120.7(1)
105.5(2)
115.6(2)
180
-0.146 (9.9%)
Wide ranges in Z12 values are not expected to affect
the k11(calc) values obtained from cross rate theory,
especially when K12 is small. All of the reactions exam-
ined for which both kii values are independently known
from NMR studies have given smaller k12 (and hence
smaller k11(calc) values) than predicted by (4). The
deviations observed for di- and pentamethylferrocene-
hydrazine reactions are rather small, and cross rate
studies of these reactions produce k11(calc) values when
analyzed using (4) which are usefully close to the
k11(NMR) values. In this work the θ ) 180° hydrazine
33N)2 was found to give a k11(calc) value comparably close
to k11(NMR) of the θ ) 0° hydrazines previously studied
and also close to that estimated completely independently
of both cross rate and NMR studies from isomerization
of 32N)2+ isomers involving electron transfer. The 33N)2
and 32N)2 systems were shown to have similar λ values
by X-ray crystallographic studies, establishing compa-
rable geometry reorganizations upon electron loss. k11
values were estimated for the θ ∼120° hydrazines 22/
RR′, which have k11 values too small to measure by any
other method reported, and that obtained for 22/iP r 2 was
shown to be consistent with its unusual heterogeneous
ET behavior.
R(CNC), deg
R
av,c deg
+7.7
θ,d deg
a
b
From ref 21. From this work, for the TsO- salt. Data quoted
are for the nondisordered cation unit, a . c Average of the three
bond angles at nitrogen. Lone pair, lone pair twist angle. e From
d
ref 20. Both oxidation states are in the conformation shown as
the structure 32N)2. f For the NO3-‚H2O salt.
The degree of structural similarity between the com-
ponents of an ET reaction does not allow accurate
prediction of how well (4) works. Anomalously low
k11(calc) values are produced when Cp *2F e is employed
as a reactant, for unknown reasons. The ratio of k11(calc)
values for 21/u 22 using Cp *Cp F e and Cp *2F e as the
other component is 8.2, which is larger than the 6.4
observed for reaction of 22/tBu Me with Cp *Cp F e+ and
TMP D+, despite the extreme difference in ET parameters
for the latter three components. Conversely, the hydra-
zine-hydrazine reaction (Table 2, reaction 8) gives a
sigificantly smaller k11(calc) than does the hydrazine-
pentamethylferrocene case (reaction 7). More hydra-
zine-hydrazine and hydrazine-TMP D reactions will
have to be studied before we can tell whether the
behavior observed in these cases has any generality.
F igu r e 2. Thermal ellipsoid drawing of the nondisordered
cation unit (a ) of 33N)2.
disordered structure of 32N)2+TsO- was obtained, which
demonstrated that its nitrogens are significantly pyr-
amidal.20 We have now obtained the structure of
33N)2+TsO- in an effort to find out how much the
geometry reorganizations upon electron removal differ
for these two θ ) 180° hydrazines. 33N)2+TsO- crystals
have two different half-molecules of cation per anion; one
cation (b) was badly disordered, fit in the solution of the
structure with a complex model having three different
nearly superimposed b units, and is not considered
further. The other half-cation unit (a ) was not disordered
and provides usable geometric information (see Table 7
and Figure 2). As can be seen from Table 7, the NN bond
length changes upon electron removal from 32N)2 and
33N)2 are within experimental error of being the same,
and the changes in pyramidality are comparable, so that
similar λin values and hence k11 values are expected.
Exp er im en ta l Section
NMR Lin e Br oa d en in g for Cp *Cp F e0/+
. Air-free mix-
tures of neutral and cation in base-washed glassware proved
stable enough to allow experiments in NMR tubes closed by
septa. Aliquots of a solution of Cp *Cp F e+BF 4- in acetonitrile
containing 0.09 M tetraethylammonium fluoroborate were
added to a solution of Cp *Cp F e0 in the same solvent, and
NMR data were recorded on a Brucker 250 MHz spectrometer
at 25 °C. The data are summarized in Table 1.
NMR Lin e Br oa d en in g for 33N)20/+. A solution of 33N)2
(20.2 mg, 0.813 mM) in 400 µL of CD2Cl2 (0.2033 M) was
treated with aliquots of a solution of 33N)2+P F 6- 8b(7.0 mg,
0.178 mmol) in 700 µL of CD2Cl2 (0.0254 M). Spectra were
recorded at 25.5 °C (determined using a methanol thermom-
Another reason for interest in using 33N)2 as a
reactant in cross rate studies is that it provides the
opportunity to study hydrazine-hydrazine ET reactions.
33N)2 has λmax ) 340 nm,22 significantly longer wave-
+
length absorption than most other hydrazine radical
(22) (a) Nelsen, S. F.; Teasley, M. F.; Kapp, D. L.; Kessel, C. R.;
Grezzo, L. A. J . Am. Chem. Soc. 1984, 106, 791. (b) Nelsen, S. F.;
Kessel, C. R.; Brien, D. J . J . Am. Chem. Soc. 1980, 102, 702.
(23) Nelsen, S. F.; Blackstock, S. C.; Yumibe, N. P.; Frigo, T. B.;
Carpenter, J . E.; Weinhold, F. J . Am. Chem. Soc. 1985, 107, 143.
(24) Van Geet, A. L. Anal. Chem. 1970, 42, 679.

1412 J . Org. Chem., Vol. 61, No. 4, 1996
Nelsen et al.
eter22) on a Brucker 250 MHz spectrometer. Amounts of
exchange line broadening were assigned by fitting the observed
spectrum to that without cation after adding various amounts
of broadening.3,10 Pairs listing the volume, µL, of cation
solution added and in parentheses dynamic broadening (Hz):
15 (3.0), 30 (5.0), 50 (8.2), 80 (11.8), 120 (16.0), 160 (20.0), 210
(24.0). These data give kex ) 8450 M-1 s-1. A separate run at
25.5 °C using 0.180 neutral and 0.020 M cation solution gave
the following: 15 (2.6), 30 (4.2), 50 (6.4), 80 (8.8), 120 (11.8),
160 (14.6), 210 (17.6), 260 (20.2); kex 8000 M-1 s-1. A variable
temperature run with a solution 0.144 M in 33N)20 and 0.0066
M in 33N)2+P F 6- gave temperature, °C, paramagnetic broad-
ening (Hz) in parentheses, kex (M-1 s-1) values in brackets:
25.5 (18.5) [8900], 6.45 (9.0) [4300], 11.1 (10.7) [5100], 15.9
(12.4) [5900], 20.6 (15.2) [7300], 25.2 (18.2) [8700], 30.0 (22.0)
[10500]; kex(25 °C) 8700, ∆Hq ) 5.8 ( 0.4 kcal/mol, ∆Sq ) -21
( 1.3 cal deg-1 mol-1. We estimate approximately a 5% error
in kex(25 °C).
fitting routines. All reactions were studied under pseudo-first-
order concentration conditions with the neutral typically in
10-fold or greater stoichiometric excess of the radical cation.
Each reaction was studied by varying the concentration of the
neutral over as wide a range as possible, typically at least a
factor of 10, and obtaining the second-order rate constant from
a linear regression of a plot of the observed pseudo-first-order
rate constants versus the neutral concentration. The reported
uncertainties reflect the uncertainty of this fit.
21
Cr ysta l Str u ctu r e of 33N)2+TsO-.28 Neutral 33N)2
(0.303 g, 1.22 mmol) was suspended in acetonitrile and cooled
to 0 °C, and a solution of silver tosylate (0.339 g, 1.21 mmol)
in 20 mL of acetonitrile was added by syringe over 2 min. After
stirring 45 min at 0 °C, filtration through Celite, and concen-
tration to one-forth of the initial volume, 30 mL of ether was
layered on top, and the product was collected after 1 day.
Recrystallization from acetonitrile (∼5 mL) followed by care-
fully layering 30 mL of ether on top in a Schlenck tube gave
0.420 g (82%) of 33N)2+TsO- as dark yellow crystals, dec 203-
204 °C. The structure was determined at 113(2) K using a
0.4 × 0.2 × 0.1 mm crystal, on a Siemens P3f diffractometer
using graphite-monochromated Cu KR radiation (λ ) 1.541 78
Å), Wyckoff scan type, θ range 2.00 to 57.00°. The solution of
the structure with direct methods used program SHELXS-86
and the refinement used SHELXL-93, which refines on F2
values.27 33N)2+TsO- (C16H28N2 + C7H7O3S, fw 419.59) crys-
tals are monoclinic, space group P21/c, unit cell dimensions a
) 9.4267(8), b ) 12.5296(13), and c ) 17.777(2) Å, â )
90.104(9)°, volume 2099.7(4) Å3, Z ) 4, density(calc) ) 1.327
mg/m3, absorption coefficient ) 1.327 mm-1, F(000) ) 908.
Reflections collected ) 3814, independent reflections 2822
[R(int) ) 0.0379], data ) 2822, restraints ) 0, parameters )
294, goodness-of-fit on F2 ) 1.042, final R indices R1/wR2 )
0.0483/0.1170, R indices (all data) R1/wR2 ) 0.0639, 0.1277,
extinction coefficient 0.0022(2), largest difference peak/hole )
0.337/-0.315 e A-3. The structure consists of two half-cations
per anion. One of the half cations (a ) is well behaved, but the
other (b) is disordered. Three orientations of the disordered
half were located, with occupancies 0.379(9), 0.334(9), and
0.287(7), and the disordered nitrogens and carbons were
refined with isotropic thermal parameters. Nearly 300 geom-
etry restraints were used in initial refinements to get the
disordered groups to behave in a chemically reasonable man-
ner. After the restrained refinements converged, the restraints
were removed for the final refinement. We have little faith
in the postions of the disordered site atoms and do not consider
these data here. The methyl of the tosylate anion was also
disordered and modeled with six 0.5 occupancy hydrogens.
Cyclic volta m m etr y experiments have been previously
described.18
Stop p ed -F low Kin etics. HPLC or spectrophotometric
grade solvents (Aldrich or Baker) were used for all kinetic
measurements. Tetrabutylammonium perchlorate (Baker or
Kodak), used to maintain ionic strength, was recrystallized
from 50/50 volume percent water/ethanol. Hydrazine solutions
were prepared by weighing ca. 5 mg samples of the neutral
hydrazine, or the tetrafluoroborate salt of its conjugate acid,
to the nearest 0.05 mg using a Mettler microbalance. These
samples were transferred to a Coy oxygen-free glovebox and
dissolved in deoxygenated solvents to which sufficient stand-
ardized sodium hydroxide solution had been added to ensure
complete conversion of the hydrazine to its neutral, conjugate
base form. These solutions were typically 1 mM in NaOH and
0.5% in water. TMP D (Aldrich) was oxidized to TMP D+ with
NOPF6 and crystallized as its tetrafluoroborate salt. TMP D+
solutions were prepared by dissolving weighed samples of
TMP D[BF 4] in acetonitrile containing tetrabutylammonium
perchlorate. Ferrocenium hexafluorophosphate salts were
prepared by literature methods from their ferrocenes.25 1,1-
Dimethylferrocene (Alfa Inorganics) was recrystallized from
ethanol, decamethylferrocene (Strem Chemical) was sublimed,
and 1,2,3,4,5-pentamethylferrocene was prepared by a modi-
fication of the method of King26 for the preparation of deca-
methylferrocene in which a mixture of equal amounts of
LiC5H5 and LiC5(CH3)5 was used in place of LiC5(CH3)5.
Ferrocenium hexafluorophosphate solutions were also pre-
pared in an inert atmosphere and their concentrations deter-
mined by spectrophotometric analysis (Cp *2F e+, ꢀ778 ) 540
M-1 cm-1, Cp ′2F e+, ꢀ650 ) 332 M-1 cm-1, Cp *Cp F e+, ꢀ740
)
360 M-1 cm-1). All solutions were prepared immediately
before use, and Cp *Cp F e+ solutions were prepared under
slightly acidic (ca. 1 × 10-5 M HCl) conditions because its
neutral or basic acetonitrile solutions decompose slowly.
Reactant solutions were transferred under an inert atmo-
sphere of nitrogen to a Durrum Model D-110 stopped-flow
spectrophotometer interfaced to a 386 computer with On-Line-
Instrument-Systems (OLIS) stopped-flow data acquisition and
analysis software. Reactions between all hydrazines and
ferrocenes were monitored by observing the absorbance de-
crease that accompanies reduction of the ferrocenium to the
ferrocene between 280 and 320 nm; reaction between 21/21+
and 33N)2 was monitored by observing the absorbance increase
at 340 nm due to oxidation of the neutral hydrazine; the
oxidation of 22/tBu Me by TMP D+ was monitored by observing
the reduction of TMP D+ absorption at 614 nm. Typically
three replicate measurements were made on each pair of
reactant solutions, and the observed absorbance changes were
fitted to the appropriate integrated rate equation using OLIS
Ack n ow led gm en t. We thank the National Insti-
tutes of Health for partial financial support of this work
under Grant GM 29549 (S.F.N.) and the National
Science Foundation under Grants CHE-8921985,
-9504133 (J .R.P), and -9105485 (S.F.N.). We thank a
referee for valuable comments on the applicability of eq
6.
J O9510090
(27) (a) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467. (b)
Sheldrick, G. M. J . Appl. Cryst. (in preparation). (c) Neutral atom
scattering factors were taken from International Tables for Crystal-
lography; Kluwer: Boston, 1992; Vol. C, Tables 6.1.1.4, 4.2.6.8, and
4.2.4.2.
(28) The author has deposited atomic coordinates for this structure
with the Cambridge Crystallographic Data Centre. The coordinates
can be obtained, on request, from the Director, Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK.
(25) Carney, M. J .; Lesniak, J . S.; Likar, M. D.; Pladziewicz, J . R.
J . Am. Chem. Soc. 1984, 106, 2565.
(26) King, R. B.; Bisnette, M. B. J . Organomet. Chem. 1967, 8, 287.